Method and apparatus for performing frequency spectrum inversion

Abstract

An apparatus and method for creating voice privacy by performing frequency spectrum inversion in electronic voice transmission systems includes the steps of digitizing an analog signal and inverting the frequency spectrum of the digitized audio signal. From the inverted spectrum, a complex signal is created from which the real component is extracted to produce a real signal suitable for transmitting. The digital signal processing is performed entirely with software. The scrambling and descrambling processes are nearly identical, therefore, the same hardware and software may be used to scramble and descramble the signal. The apparatus may uses a "rolling code" to increase the effectiveness of the scrambling.

What is claimed is: 
     
       1. A method of processing a digitized audio signal having upper and lower sidebands comprising the steps of: inverting the frequency spectrum of at least one of the sidebands of the digitized audio signal;   constructing a complex signal having real and imaginary components and a single sided frequency spectrum based on the inverted frequency spectrum; and   extracting the real component of the complex signal to produce a real signal.   
     
     
       2. The method of claim 1 wherein the step of inverting the frequency spectrum further comprises the step of frequency shifting the signal to shift the lower sideband of the signal to the upper sideband. 
     
     
       3. The method of claim 1 wherein the step of inverting the frequency spectrum further comprises the steps of: applying a frequency shift to the signal to center the lower sideband of the signal about a frequency near zero;   filtering out the upper sideband of the signal; and   applying a second frequency shift to construct the complex signal.   
     
     
       4. The method of claim 3 wherein the second frequency shift is variable. 
     
     
       5. The method of claim 3 wherein the second frequency shift is varied over time to create a rolling code. 
     
     
       6. The method of claim 3 further comprising the step of converting the real signal to an analog signal. 
     
     
       7. The method of claim 6 further comprising the step of transmitting the analog signal. 
     
     
       8. A method for creating voice privacy in electronic voice transmission systems comprising the steps of: (a) scrambling a voiced audio message by; (a1) digitizing an analog representation of the message,   (a2) inverting the spectrum of the digitized representation of the message, wherein the digitized representation of the scrambled audio message has real and imaginary parts and is inverted by: (a2.1) translating the lower sideband of the digitized audio signal to create a complex baseband signal;   (a2.2) filtering out the upper sideband;     frequency shifting the complex baseband signal; and (a2.3) extracting the real part of the signal to produce a real signal     (b) creating a scrambled audio message based on the inverted spectral representation of the message; and   (c) transmitting the scrambled audio message.     
     
     
       9. The method of claim 8 wherein the steps of inverting the spectrum of the digitized representation of the message is performed with software. 
     
     
       10. The method of claim 8 further comprising the steps of: receiving the scrambled audio message;   descrambling the scrambled audio message by creating a digitized analog representation of the scrambled audio message, inverting the spectrum of the digitized analog representation of the scrambled audio message, and creating an unscrambled audio message based on the inverted spectral representation of the scrambled audio message.   
     
     
       11. The method of claim 8 wherein the digitized analog representation of the message is inverted by frequency shifting the lower sideband of the digitized analog representation of the message to 0 Hz while filtering out the upper sideband of the of the digitized analog representation of the message. 
     
     
       12. The method of claim 11 further comprising the step of frequency shifting the lower sideband of the digitized analog representation of the message a second time after the filtering to move the remaining signal spectrum to the upper sideband. 
     
     
       13. The method of claim 12 wherein the digitized analog representation has real and imaginary portions, and further comprising the step of extracting the real portion of the digitized analog representation. 
     
     
       14. A method of scrambling and descrambling an audio signal comprising the steps of: (a) converting an analog audio signal to a digitized audio signal having real and imaginary parts, and upper and lower sideband components;   (b) frequency shifting the digitized audio signal such that the lower sideband components are centered to create a complex baseband signal;   (c) filtering out the upper sideband components of the digitized audio signal;   (d) shifting the filtered complex baseband signal such that the filtered complex baseband signal is centered at a desired frequency in the upper sideband;   (e) extracting the real part of the signal to produce a real signal;   (f) converting the real signal to an analog scrambled audio signal;   (g) repeating steps (a) through (e) with the scrambled analog audio signal; and   (h) converting the real signal to an analog descrambled audio signal.   
     
     
       15. The method of claim 14 further comprising the steps of (f1) and (f2) performed after step (f), where steps (f1) and (f2) are: (f1) transmitting the analog scrambled audio signal; and   (f2) receiving the transmitted analog scrambled audio signal.   
     
     
       16. An apparatus for processing an audio signal comprising: (a) an analog to digital converter for sampling and digitizing an audio signal;   (b) a processor connected to the analog to digital converter, said processor performing the processing steps of inverting the digitized audio signal;   (c) the processor performing the processing steps of inverting the digitized audio signal having real and imaginary components by: (c1) shifting the digitized signal to center the negative frequency components of the frequency spectrum of the digitized signal to create a complex baseband signal,   (c2) filtering out the upper sideband of the complex baseband signal,   (c3) shifting the complex baseband signal to the upper sideband to center the complex baseband signal in a desired positive frequency band, and   (c4) extracting the real part of the signal to produce a real audio signal; and     (d) a digital to analog converter connected to the processor to convert the digitized audio signal to an analog signal.   
     
     
       17. The apparatus of claim 16 further comprising a transmitter connected to the processor to transmit the analog signal.